Free Astronomy Magazine July-August 2021

30 JULY-AUGUST 2021 Earth’s Trojans. It seems unrealistic that these populations of small as- teroids could have been character- ized so precisely by possible alien astronomers, to the point of plan- ning and making a landing on one of them, in a position that allows the study of our planet. Inserting a probe into Earth’s orbit would un- doubtedly be less complicated and more informative. A subsequent de- orbiting with entry into the atmos- phere would have prudently erased all traces of the visit. Less discreet aliens could have placed observation instruments on the Moon, certainly well known to those who are able to observe the Earth from a few light- years away. A civilization with ichnoscale slightly higher than ours would already be able to do all this, and we cannot entirely exclude the presence of alien technosignatures on the lunar surface. In fact, despite the intense and continuous mapping of our nat- H ypothetical megastructures of planetary size could be detected due to the anomalous heat dispersion revealed compared to a natural rocky body. Alongside, some “hybridized” moons. [TechnoClimes 2020] of finding, in heliocentric orbit or on the surfaces of rocky bodies, arti- facts sent into our solar system by one or more alien civilizations. It seems very unlikely that such finds could be made, but the basic idea is not so crazy. We know well that the stars are not fixed with respect to each other and that in their motion around the center of the galaxy they can occasionally “graze” each other. It has been calculated that, on aver- age, every 100,000 years a star tran- sits less than a light-year from the Sun, penetrating the Oort Cloud. This implies that since life was born on Earth, some tens of thousands of stars have passed at a close enough distance to allow any technological civilizations they host to notice the chemical “contamination” of our at- mosphere, caused precisely by the presence of life forms. Just as we are planning to send a probe into the probably sterile Proxima Centauri planetary system, over four light- years away, it is conceivable that others have planned a similar mis- sion when, passing even closer to our solar system, they realized that on the third planet there was some- thing far more vital. Some TechnoClimes 2020 attendees believe that if we have received vis- its from alien probes, they may still be somewhere in our planetary sys- tem, albeit likely inactive and de- teriorated. The same researchers suggest that if the aliens’ goal was to study the Earth and its inhabi- tants (in a broad sense) for a rela- tively long period, those wrecks could be either on the Moon or on asteroids with orbits closer to that of our planet, such as the NEOs and the